1. Field of the Invention
The present invention relates to ceramic preforms, to a process for the preparation thereof and to the use of such preforms, in particular, for the production of metal or ceramic composite shaped articles.
2. Description of the prior Art
The production of metal composites is known to this art. Compare the process described in EP-A-0,328,805, according to which a metal in the molten state is introduced into a cavity, a preform is placed on the molten metal and, under the influence of pressure applied to the preform, the molten metal impregnates into said preform. According to this patent, the preform consists of a refractory fibrous material such as alumina, zirconium dioxide, silica, silicon carbide, silicon nitride or titanium diboride in the form of chopped fibers.
EP-A-0,337,732 describes the production of metal composites reinforced with B-type silicon nitride whiskers. This process entails impregnating the whiskers with molten metal and solidifying the latter to provide a metal composite reinforced by such whiskers. The whiskers are themselves obtained by high-temperature calcination of an amorphous or .alpha.-type silicon nitride powder. It is in the form of fibers having a diameter of 0.1 to 5 .mu.m and a length of 2 to 100 .mu.m.
Japanese Application 1,180,929 describes the preparation of preforms for aluminum-based composites by compression of short fibers of alumina, silica, silicon nitride, silicon carbide, glass or carbon, to which a lubricant such as graphite, MoS.sub.2, WS.sub.2, Sn, Pb, alumina cement, gypsum or fluorite, a resinous binder and a solvent have been added.
Japanese Application 1,157,803 describes the preparation of preforms by mixing a fine inorganic powder (aluminum oxide, silicon carbide, silicate, titanium dioxide, zirconium oxide, silicon nitride, boron carbide or boron nitride) with a binder selected from among water-soluble resins, malaxating the entire mass, and introducing it into a mold previously lined with a sheet of polyvinyl acetal and this assembly is heated to solidify and integrate the acetal sheet on the surface of the preform.
Japanese Application 1,225,732 also describes the impregnation of short fibers (for example Al.sub.2 O.sub.3) using a metal matrix. These preforms contain up to 10% of an inorganic binder and up to 5% of silica particles.
Japanese Application 87-139,838 describes the production of metal composites containing silicon carbide whiskers of precise and uniform shape.
Japanese Application 87-299,569 describes the production of metal composites (Al) from a preform of inorganic fibers. These fibers are used in the form of a felt obtained by immersion of woven SiC, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, C and/or metal fibers in a bath agitated by ultrasound and containing ceramic, carbon and/or metal short fibers, whiskers and/or powders.
It will thus be seen that the known preforms are essentially comprised of fibrous materials or whiskers or powder of a metal oxide, carbide or nitride, and consolidated by means of a water-soluble resin of the polyvinyl acetate, cellulose acetate or polyvinyl alcohol type, or starch.